Java: HashSet Vs TreeSet – Difference between HashSet and TreeSet in Java

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While searching solution for my Java exception online found very nice explanation on StackOverflow about some basic difference between HashSet and TreeSet.

HashSet is much faster than TreeSet (constant-time versus log-time for most operations like add, remove and contains) but offers no ordering guarantees like TreeSet.

HashSet:

  • class offers constant time performance for the basic operations (add, remove, contains and size).
  • it does not guarantee that the order of elements will remain constant over time
  • iteration performance depends on the initial capacity and the load factor of the HashSet.
    • It’s quite safe to accept default load factor but you may want to specify an initial capacity that’s about twice the size to which you expect the set to grow.

TreeSet:

  • guarantees log(n) time cost for the basic operations (add, remove and contains).
  • guarantees that elements of set will be sorted (ascending, natural, or the one specified by you via it’s constructor).
  • doesn’t offer any tuning parameters for iteration performance.
  • offers a few handy methods to deal with the ordered set like first(), last(), headSet(), and tailSet() etc.
HashSet Vs TreeSet

Important points:

  • Both guarantee duplicate-free collection of elements
  • It is generally faster to add elements to the HashSet and then convert the collection to a TreeSet for a duplicate-free sorted traversal.
  • None of these implementation are synchronized. That is if multiple threads access a set concurrently, and at least one of the threads modifies the set, it must be synchronized externally.
  • LinkedHashSet is in some sense intermediate between HashSet and TreeSet. Implemented as a HashTable with a linked list running through it, however it provides insertion-ordered iteration which is not same as sorted traversal guaranteed by TreeSet.

So choice of usage depends entirely on your needs but I feel that even if you need an ordered collection then you should still prefer HashSet to create the Set and then convert it into TreeSet.

e.g. Set<String> s = new TreeSet<String>(hashSet);

Some more Java Tutorials to look: https://crunchify.com/category/java-tutorials/

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